4. Coolest thing I’ve seen all week? The BBC’s How Many Really?, a website that lets you compare the number of people involved in a major historical event with the number of people you know via things like Facebook and Twitter. (If you don’t want to link to your social media accounts, just click on the “Add Your Own Numbers” button and pick a random item like a bus or a 747.) Fascinating. (Hat tip: The Boston Globe’s Brainiac blog)